This invention relates to a clutch system of the friction type placed in a power transmission system. Typical clutch systems include a clutch input such as a clutch basket, a clutch output such as a center clutch, and one or more plates making up a clutch pack and disposed between the clutch input and clutch output. When the clutch pack is compressed, the clutch input and clutch output become rotationally coupled. The clutch pack is typically compressed by a pressure plate; the pressure plate typically providing a compressive force via a spring mechanism or through a centrifugally actuated mechanism.
Typically, such clutch systems include a clutch disengagement system consisting of a clutch lever mechanically coupled to the pressure plate such that when the clutch lever is actuated, the pressure plate's compressive force on the clutch pack is removed, disconnecting the rotational coupling between the clutch input and clutch output. Clutch disengagement systems typically couple the clutch lever to the pressure plate mechanically through a hydraulic actuation system or a cable actuation system. The clutch system incorporated in a 2011 Honda CRF-450R represents a typical prior art clutch system.
Most motorcycles incorporate a manual transmission coupled to the engine via a multi-plate clutch assembly. Typically, the multi-plate clutch is engaged/disengaged by the driver via a clutch lever mounted on the handlebar. Although the clutch lever operated clutch allows the driver to control the clutch engagement/disengagement, often times motorcycle drivers find the clutch lever difficult to operate smoothly. New riders have difficulty adjusting to smoothly engaging the clutch while operating the throttle to move the vehicle from a standing start. Experienced riders may need to partially disengage the clutch when traveling slowly to allow the engine to continue running without stalling. Motorcycle racers often have a difficult time controlling the engagement of the clutch and the application of the throttle to maximize acceleration. Off-road motorcycle racers often need to stop the rear wheel suddenly with the rear brake, causing the engine to stall if the clutch is not first disengaged. An automatic clutch can help overcome many of the problems associated with a manual clutch.
U.S. patent application Ser. No. 12/412,245 discloses an automatic clutch system incorporating an expanding friction disk and is incorporated herein by reference.
U.S. patent application Ser. No. 13/338,535 discloses a wear liner and improved friction disks and is incorporated herein by reference.
U.S. patent application Ser. No. 14/094,144 discloses an improved friction disk design and is incorporated herein by reference.
The prior art shows expanding friction disk designs comprised of centrifugal weights housed between a first plate and a second plate where the first plate and second plate are compressed together with biasing springs wherein the centrifugal weights, first plate and second plate contain features which provide a sliding contact interface between them with at least one side of the interface being a ramp mechanism. The ramp mechanism provides the relative lift to create expansion when the centrifugal weights move outwardly and overcome the biasing springs due to centrifugal force. In some designs the centrifugal weights are ball bearings which slide in grooves formed into either the first plate or second plate.
Although sliding contact between these parts and the respective ramp features provides good performance, there are application and design limitations associated with incorporating sliding contact between the centrifugal weights, first plate and second plate. These limitations include losses due to friction from the surface on surface sliding contact interfaces which can make optimizing the engagement and disengagement RPM difficult. In some cases the friction between the sliding contact areas is accentuated due to the presence of static and dynamic coefficient of friction differences inherent with sliding bearing interfaces. The friction loss and static to dynamic coefficient of friction transition can force the user to run an engagement point significantly higher than normal idle speed so that the disengagement point does not cause the engine to stall as the centrifugal weights return to their collapsed positions when engine RPM drops to idle. The engagement point is the RPM threshold where the centrifugal force of the centrifugal weights overcomes the biasing springs causing the expanding friction disk to expand causing the clutch pack and pressure plate to be pressurize resulting in the vehicle beginning to move. When the engagement point exceeds approximately 200 RPM above normal idle speed, the compression braking relied upon by many users is not adequate and a free-wheel affect can take place when the centrifugal weights return fully to their starting position and the clutch pack and pressure plate are no longer pressurized.
The static to dynamic coefficient of friction transition can also cause abrupt clutch engagement, which in certain terrain situations can make controlling power delivery from the engine to the drive train difficult resulting in a loss of traction at the driving wheel or wheels. This abruptness is preempted by the user having to rev the engine to a higher RPM than what the user would expect making it difficult to adapt to using the expanding friction disk while operating the vehicle.
The prior art shows the use of two vertically stacked ball bearings to provide rolling contact between adjacent parts. Although two vertically stacked ball bearings resolves the friction issues inherent with a sliding surface contact, the design is prone to wearing the mating features in the respective parts. Applied to an expanding friction disk, the parts prone to wear are the first plate and second plate which are in contact with the two vertically stacked ball bearings. When the first plate and second plate wear, the function of the expanding friction disk is degraded leading to clutch failure.
Therefore a need exists for an improved expanding friction disk assembly design which reduces friction specifically the difference between static and dynamic coefficient of friction at the base side and centrifugal weight interfaces and eliminates premature wear increasing the useful life of the expanding friction disk.
An expanding friction disk is significantly thicker than the standard friction disk it is intended to replace. Therefore, when using an expanding friction disk it is required to replace other clutch parts to accommodate the increased thickness of the expanding friction disk in order to maintain the same total number of friction surfaces as the original clutch included with the vehicle. The friction surfaces in a clutch correspond to the adjacent faces in contact between friction disks and drive plates which are interleaved within a clutch pack. Maintaining the same number of friction surfaces allows the compressive force transmitted through the pressure plate to remain the same and therefore maintains the same clutch lever pull force for the user while maintaining the same torque capacity within the clutch system.
Alternatively, an expanding friction disk can be used without replacing other clutch parts to accommodate the increased thickness of the expanding friction disk. In this situation additional friction disks and drive plates are removed to accommodate the increased thickness of the expanding friction disk while maintaining the same relative overall clutch pack height. Maintaining approximately the same overall clutch pack height is important in not disrupting the function of the clutch lever activated engagement/disengagement mechanism. With the additional friction disks and drive plates removed, the user is required to increase the compressive force transmitted through the pressure plate in order to maintain the same torque capacity as the original clutch included in the vehicle to counter-act the loss of friction surfaces. Increasing the compressive force transmitted through the pressure plate results in additional clutch lever pull force for the user which is undesirable as it makes the clutch lever more difficult to use when controlling the power delivery of the engine to the drive train. This clutch pack, with reduced surfaces, is also prone to overheating and slip which is undesirable.
Therefore a need exists for an expanding friction disk which is compatible with improved frictions disks of reduced thickness allowing the number of friction surfaces, clutch pack height and compressive force for the clutch system to be maintained.
It is therefore an object of the present invention to provide an improved expanding friction disk which operates with minimal friction and minimal difference between static and dynamic friction, has increased useful life by minimizing wear between centrifugal weights and the first plate and second plate, and which can be configured to operate with improved friction disks which utilize a wear liner to protect the clutch input.